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No technology is perfect, however. And factors such as human error, genetic mutations, mixed samples and contaminants can prevent an accurate DNA profile being obtained, say forensic scientists.
In the Northern Territory, a recent DNA profile of a man wrongly indicated he was a woman. The man was found to have a rare genetic mutation which interfered with the routine gender test.
The mistake was obvious in this case, says Dr Paul Roffey, of Charles Sturt University. But in some investigations of unidentified crime samples, or when knowing the sex of an offender is crucial, a second, different gender test should be considered, he says.
Scientists at the Forensic Biology Laboratory in Perth working on the case of a missing young woman have been able to obtain her DNA profile from saliva or other cells left in two of her old lipsticks.
But they also found from their research there was a "real danger" that chemicals in lipstick could interfere with routine DNA analysis. Mouth swabs - to get saliva for DNA testing - need to be done very carefully to avoid lipstick contamination, they say.
In a bizarre mix-up in New Zealand, an innocent victim of a bashing became a double murder suspect, following bungled DNA profiling at the Institute of Environmental Science and Research laboratory in Auckland.
The man was bashed outside a pub in Christchurch on the South Island in April 1998, and his blood was sent for analysis.
Two months earlier a man had been killed in his Wellington home on the North Island - a crime later admitted to by Kuka Tiai.
Tiai told police he had acted alone, so they were concerned when samples of DNA from under the dead man's fingernails and from a bloodspot in his bedroom did not match that of Tiai nor the deceased.
They got the laboratory to search their other cases, and a match was made with the bashed man's DNA profile. To make matters worse, blood collected from a roller door at a second murder - a Wellington gangland killing - also had a DNA profile identical to that of the bashed man.
The police made "extensive inquiries" to see if he could have been involved in either of the Wellington homicides, says a government-commissioned report made public last month.
Luckily his movements were trackable, because he had used his EFTPOS card regularly, and he was cleared of suspicion.
The man was never in jeopardy of being charged, let alone convicted, say the report's authors, a former NZ chief justice, Sir Thomas Eichelbaum, and the president of the Royal Society, Sir John Scott.
But even the NZ Justice Minister, Phil Goff, noted this may not have been the case if the bashed man had been a gang member in Wellington.
The high-powered inquiry - the third into the case - could not identify how the mix-up occurred. It concluded there was probably "accidental contamination" of the samples at the laboratory.
"Accidental contamination ... is a constant hazard in DNA laboratories," report Eichelbaum and Scott. "But the event ought not to be regarded as casting doubt on the efficacy of DNA testing as an investigative tool."
The Australian prisoners' rights group, Justice Action, claims the NZ case is one of many overseas that demonstrate the unreliability of DNA profiling.
But the director of the National Institute of Forensic Science, Dr Alastair Ross, says safeguards have been introduced to ensure the integrity of DNA databases in Australia. Laboratories must meet strict operational guidelines, and a new system of accrediting individual forensic scientists has been introduced.
If a match is obtained between a DNA profile and DNA from a crime scene, this will not be used in court. Police must take a new sample, as occurred with the man charged for the Wee Waa rape, for testing.
DNA evidence is only one element of a criminal investigation. And the NZ experience could be interpreted as showing the system works, says Ross. "It showed that, in the end, he wasn't the person."
Chaseling has individually compared DNA profiles from 5,500 Australians, including Northern Territory politicians, offenders and informed Red Cross blood donors.
Only two people had identical DNA at seven of the nine regions, or loci. None had an identical DNA profile when eight or nine loci were compared, she says. More than 20 loci can be checked if there are any doubts, and 13 are routinely used for DNA profiling in the US.
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